Atrial fibrillation (AF) has typically been treated using shock therapy. When a patient's heart is found to be in AF, a jolting electrical pulse, or shock pulse, is delivered to the patient in order to reactivate the electrical signals throughout the heart. The shock pulse may be administered via external defibrillators, or via implantable cardiac device (ICDs) configured to deliver such a shock. Patients, however, feel uncomfortable using shock therapy because of the pain and discomfort associated with the shock. This is particularly the case if the shock therapy is applied frequently due to the reoccurring nature of AF.
Pacing therapies, such as anti-tachycardia pacing (ATP), have been proposed for the treatment of AF. Pacing therapy, if successful, is preferred over shock therapy since pacing therapy is relatively pain free and saves battery consumption. ATP therapies used to treat AF generally consist of “underdrive” pacing and “overdrive” pacing. Underdrive pacing is when the heart is paced at a rate slower than the arrhythmia rate. Overdrive pacing is when the heart is paced at a rate faster than the arrhythmia rate. Subsequently, overdrive pacing consists of various forms of pacing; i.e. scanning burst, shifting bursts, ramp pacing, etc.
Even though significant progress has been made with pacing therapy, however, it is still limited primarily to the prevention of AF and termination of atrial flutter. If AF can be terminated with pacing therapy, with or without pharmacological assistance, it will be a significant improvement in the treatment of AF.
For successful treatment of AF, an important aspect is whether the cardiac tissue can be captured. When cardiac tissue is captured, a temporary conduction block zone is created, which hinders conduction of fibrillatory wave propagation. If enough of these conductive blocks are created, fibrillatory waves will run out of tissue to propagate, thus extinguishing arrhythmia. Recent studies have found that AF can be classified as one of four types, with certain types being more capturable than others. Further research suggests that atrial tissue can be captured, and more specifically, atrial tissue in Type I AF or Type II AF is more likely to be captured than Type III AF or Type IV AF.
In “Characterization of Atrial Fibrillation in Man: Studies Following Open Heart Surgery,” by J. Wells, PACE, Vol. 1, 1978, which is hereby incorporated by reference, Wells classified AF according to electrogram (EGM) morphologies. Type I AF represents discrete organized atrial EGMs with intervening isoelectric lines free of perturbation. Type II AF also represents discrete organized atrial EGMs, but without clear isoelectric intervals. Type III AF has disorganized atrial EGMs, without clear isoelectric intervals. Type IV AF is characterized as alternations between Type I AF and Type III AF.